Turbofan engine having core supercharging stage

ABSTRACT

A turbofan configuration for a gas turbine engine is disclosed. Various construction details which improve engine performance by supercharging working medium gases to the engine core are discussed. Engines configured in accordance with the present invention include an island splitter which is disposed across the fan flow path. The island splitter is spaced apart from the core engine case. A supercharging, compression stage is driven commonly with the fan stage and extends outwardly into proximity with the core case.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to turbofan gas turbine engines, and morespecifically to supercharging of working medium gases flowed through thecore section of such an engine.

2. Description of the Prior Art

Turbofan, gas turbine engines are the type of powerplants most widelyused in large aircraft today. In turbofan engines, as distinguished fromturbojet engines, a portion of the working medium is pumped axiallythrough one or more compression stages and is exhausted to theatmosphere without passing through the core section of the engine. Suchcompression stages are called fan stages.

In the least complex of turbofan engines, the stages of the fan and thestages of the core are driven by separately rotating shafts. The shaftdriving the fan stages rotates at a speed slower than the shaft drivingthe core stages. The ratio of the air flowing through the fan stagesalone to the air flowing through the core stages is referred to as thebypass ratio. The bypass ratio may be a different value for eachindividual engine model according to the performance requirements ofthat powerplant. In all turbofan engines, however, the fan stages make asubstantial contribution to the total engine thrust at take-off.

For large thrust contributions a bypass ratio of five (5) or greater istypical. At these bypass ratios the diameter of the fan need be verylarge to pass the required amount of working medium. In such aconfiguration, the root region of each fan blade is of necessity closelyspaced to the root region of the adjacent blade in order that the tipregions of the blades are not excessively spaced. The root regions ofthe blades have a relatively short chord length and a minimal twist.Also, the blade speed relative to the incoming medium is significantlyless in the root region. Resultantly, the root portions of the bladeshave a limited capacity to raise the pressure of the medium pumpedthereby. In a typical turbofan engine, the pressure ratio attainableacross the fan blades in the root region is only a ratio of about oneand five tenths (1.5) in contrast to the pressure ratio attainableacross the tip regions of the blades which is a ratio of approximatelyone and seven tenths (1.7).

To compensate for the reduced capacity of the fan blades in the rootregions, modern engines utilize one of two configurations for raisingthe pressure of the medium approaching the core stages of the engine. Afirst approach is embodied in representative U.S. Pat. No. 3,283,995 toFligg, Jr. entitled "Splitter Vane Construction for Turbofan Engine". Inthis turbofan configuration, low compression or supercharging stages aremechanically coupled to the fan stage immediately downstream of the fanstage. A core case circumscribes the low compression stages and extendsinto close proximity to fan blades. The inward portion of the workingmedium gases discharged by the fan stage is captured by the core caseand is directed into the low compression stages.

In a second approach illustrated by U.S. Pat. Nos. 3,494,129 to Krebs etal entitled "Fluid Compressors and Turbofan Engines Employing Same";3,528,246 to Fischer entitled "Fan Arrangement for High Bypass RatioTurbofan Engine"; and 3,536,414 to Smith, Jr. entitled "Vanes forTurning Fluid Flow in an Annular Duct", short fan blades extendoutwardly to a flow splitter disposed across the fan stream. The flowsplitter extends into close proximity to the fan blades. Flow treated bythe inward portions of the fan blades is confined by the splitter andsubsequently treated in total by the short blades.

Although the two turbofan configurations discussed above havesignificant structural dissimilarities, both configurationsaerodynamically couple the low compression, or supercharging stages tothe fan stages. In the former, the core case extends into closeproximity with the fan blades causing the working medium discharged bythe inward portion of the fan stage to be directed in total into the lowcompression stages. In the latter, the flow splitter extends into closeproximity with the fan blades causing the working medium discharged bythe fan inwardly of the splitter to be directed in total into the shortblades forming the supercharging, compression stages. Such seriestreatment of common working medium gases is referred to as "aerodynamiccoupling", and in both configurations the supercharging stage isaerodynamically coupled to the fan stage.

Although the configurations represented by this prior art are hightechnology structures providing adequate service in th aviation fieldtoday, scientists and engineers continue to search for yet improvedconfigurations which will enhance performance and product reliability.

SUMMARY OF THE INVENTION

The primary aim of the present invention is to provide an improvedturbofan configuration for an aircraft, gas turbine engine. Improvedaerodynamic performance and engine reliabiity are sought, and specificobjects are to provide structure having enhanced tolerance of inletdistortion and of bypass duct distortion.

According to the present invention the working medium gases flowing intothe core section of a multi-spool, turbofan engine are supercharged by acore compression stage which is mechanically coupled to the fan stage,yet which is aerodynamically coupled to downstream compression stages ofthe core section.

In accordance with one detailed embodiment of the invention a coreengine case circumscribes the core compression stages and an islandsplitter spaced apart from the core case is positioned axially betweenthe fan and supercharging stages.

A primary feature of the present invention is the aerodynamicallydisassociated, but mechanically coupled fan and supercharging,compression stages. The fan stage is circumscribed by a fan case; thesupercharging, compression stage is circumscribed by a core engine case.An island splitter is disposed across the fan flow path at a locationupstream of the core case and is spaced apart from the core case. In onedetailed embodiment a plurality of guide vanes extend inwardly from theisland splitter and a larger plurality of vanes extend outwardly fromthe island splitter.

A principal advantage of the present invention is the aerodynamicdisassociation of the supercharging, compression stage from the fanstage. The supercharging stage works solely upon medium gases acceptableby the engine core and avoids work on medium gases subsequentlyexhausted to the bypass stream. Reduced sensitivity to inlet distortionsis provided by spacing the supercharging, compression stage well apartfrom the fan stage and by interposing the island splitter between thesupercharging and fan stages. Reduced sensitivity to local backpressuresin the bypass stream is provided by confining the supercharging,compression stage within the core stream. Reduced engine weight resultsfrom locating the supercharging stage in the core flow path rather thanin the more radially outward fan flow path. Performance deterioration inthe core compression stages is mitigated by enabling foreign particlesto be centrifuged outward through the space between the island splitterand the core engine case. Noise control advantages result fromconfinement of supercharging stage work within the core engine case andfrom disposition of the island splitter and associated vanes across thefan stream.

The foregoing, and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of the preferred embodiment thereof as shown in theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

A simplified, side elevation view of a turbofan, gas turbine enginehaving portions of the fan section and core section broken away toreveal fan and compression stages configured to the present invention isshown.

DETAILED DESCRIPTION

A multi-spool, turbofan engine configured to one embodiment of thepresent invention is illustrated in the Drawing. The engine principallycomprises a fan section 10 having an essentially cylindrical fan case 12and a core section 14 having a core case 16. A fan flow path 18 forworking medium gases extends axially through the fan section. A coreflow path 20 for working medium gases extends axially through the coresection. A bypass flow path 22 for working medium gases exhausteddirectly to the atmosphere from the fan flow path extends axiallybetween the core case and the fan case. A core splitter 24 for dividingthe medium gases of the core flow path from the medium gases of thebypass flow path is formed at the upstream end of the core case.

A first rotor 26 extends axially through the core section 14 and intothe fan section 10. At least one row of fan blades, as represented bythe single fan blade 28, extends across the flow path 18 into proximitywith case 12 to form a fan stage. Each blade has a tip region 30 and aroot region 32. An island spliter 34 is disposed across the fan flowpath downstream of the fan blades at a distance from the blades of lessthan one-half (1/2) chord length of the blades. The island splitterdivides the fan flow path into an inward portion 36 and an outwardportion 38. A first plurality of vanes, as represented by the singleoutward vane 40, extend between the island splitter and the fan case. Asecond plurality of vanes, as represented by the single inward vane 42,extend inwardly from the island spliter. In the embodiment shown, theinward and outward vanes are canted in the downstream direction at theoutward ends thereof.

At least one row of supercharging, compressor blades, as represented bythe single supercharging blade 44, is mechanically coupled to the fanstage of the first rotor and extends outwardly across the core flow path20 into proximity with the core case 16 to form a supercharging,compression stage. A second rotor 46 extends through the core sectionand includes at least one row of core, compressor blades, as representedby the single core blades 48, extending outwardly across the core flowpath into proximity with the core case at a location downstream of thesupercharging, compression stage of the first rotor to form corecompression stages.

During operation of the engine described, working medium gases are firstcompressed by the fan stage. Medium gases discharging from the fanblades to the inward portion 36 of the fan flow path have an averagepressure of approximately one and five tenths (1.5) times the pressureof the medium upstream of the fan blades. Medium gases discharging fromthe fan blades to the outward portion 38 of the fan flow path have anaverage pressure of approximately one and seven tenths (1.7) times thepressure of the medium upstream of the fan blades. The island splitter34 separating the inward portion of the fan flow path from the outwardportion of the fan flow path extends to less than one-half (1/2) chordlength of the fan blades from the fan blades. Working medium gasesflowing axially beneath the island splitter admix in the inward portionof the fan flow path to dissipate aerodynamic perturbations passed bythe fan blades into the fan flow path from the engine inlet. The inwardvanes 42 further smooth the flowing medium before additional work isperformed by downstream compression stages. Accordingly, thesupercharging blades treat a near homogeneous flow which is relativelyfree of pressure fluctuations.

Spacing the island splitter 34 apart from the core splitter 24aerodynamically disassociates the supercharging blades 44 from the fanblades 28. The supercharging blades 44 work on only such a portion ofthe working medium gases as is acceptable by the core compressionstages. During transient operating conditions when reduced flow iscalled for by the core, flow across the supercharging stage is alsoreduced. The fan blades, however, continue to operate on the full mediumapproaching the fan section. The excess medium in the inward portion ofthe fan flow path is allowed to flow between the downstream edge of theisland splitter and the upstream edge of the core splitter and into thebypass flow path. Such enabled operation is particularly advantageouswhere reduced flow is called for by the core stages and whereaerodynamic perturbations entrained in flow to the core are likely toinduce a stall condition over the compression blading.

Although the supercharging, compression stage is mechanically coupled tothe rotor driving the fan stage, the supercharging stage isaerodynamically coupled to the compression stages of the second rotor.The supercharging stage only does work on so much of the working mediumgases as are acceptable by the core compression stages. Wastefuldischarge of energy into the fanstream is avoided and the superchargingstage need not be capable of supplying work in excess of that requiredby the core.

One other important advantage of an engine configured to the presentconcepts is the limited exposure of the supercharging and corecompression stage to foreign matter ingested by the engine. Spacing theisland spliter remotely from the core splitter enables the outwarddirection of foreign particles through the space between the islandsplitter and the core splitter. Impact or errosion damage upon thesupercharging and core blades is reduced.

The confinement of the supercharging, compression stage within the corecase has a collateral benefit of reducing supercharging bladesensitivity to local backpressures generated in the bypass flow path.Struts across the bypass flow path, such as those illustrated in theDrawing, for supporting internal structure of the engine and forcontaining accessory drive mechanisms tends to generate local regions ofbackpressure. The supercharging blades of the present structure areisolated from such backpressure regions.

Although the invention has been shown and described with respect topreferred embodiments thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and thescope of the invention.

Having thus described typical embodiments of my invention, that which Iclaim as new and desire to secure by Letters Patent of the United Statesis:
 1. In a turbofan engine of the type having a bypass flow path and acore flow path which are separated by a core case and of the type havinga fan stage upstream of the core case, the improvement comprising incombination:an island splitter spaced radially apart from said core caseat a locaton downstream of the fan stage placing said core flow path incommunication with said bypass flow path immediately downstream of theisland splitter; and a supercharging stage mechanically coupled to thefan stage and extending outwardly into proximity with the core case suchthat the mechanically coupled supercharging stage is adapted to workonly upon medium gases of the core flow path.
 2. The invention accordingto claim 1 which includes one or more additional of said superchargingstages mechanically coupled to the fan stage and extending outwardly toproximity with the core case.
 3. The invention according to claims 1 or2 wherein said island splitter is spaced radially outward of said corecase.
 4. The invention according to claim 3 which further includes afirst plurality of vanes extending radially inward from said islandsplitter and a second plurality of vanes extending radially outward fromsaid island splitter wherein the number of outwardly extending vanes isgreater than the number of inwardly extending vanes.
 5. The inventionaccording to claim 4 wherein the fan stage has a plurality of blades,each having a chord length, and wherein the island spliter has anupstream end extending to a distance of less than one-half (1/2) fanblade chord length of the fan stage.
 6. The invention according to claim5 wherein the vanes extending radially inwardly and outwardly from theisland splitter are canted in the downstream direction at the outwardends thereof.
 7. A gas turbine engine structure comprising: a fansection including a first rotor having at least one row of outwardlyextending fan blades and including an island splitter disposedimmediately downstream of said fan blades; and a core section includingat least one row of outwardly extending, first compressor blades whichare positioned downstream of the island splitter and which aremechanically coupled to said first rotor, and including a second rotorhaving at least one row of outwardly extending second compressor bladeswhich are positioned downstream of said first compressor blades whereinsaid core section further includes a core case spaced radially apartfrom said island splitter and circumscribing the rows of compressorblades to aerodynamically couple the first and second compressor blades.